1. Field of the Invention
This invention relates to a stencil printer and a method of accurately stopping the printing drum of the printer in a predetermined position.
2. Description of the Related Art
In a stencil printer, a stencil master is wrapped around a printing drum and the image on the stencil master is printed on printing sheets by supplying printing sheets to the printing drum while rotating the printing drum.
In order to print the image on the stencil master in a desired position on the printing sheet, it is required to wrap the stencil master in place on the printing drum. For this purpose, it is required that the printing drum is constantly stopped in a predetermined position prior to wrapping the stencil master. That is, the stencil master is fed so that the leading end of the stencil master is positioned in a clamping position where a clamper on the printing drum clamps the leading end of the stencil master, and accordingly the printing drum must be stopped so that the clamper is correctly positioned in the clamping position. For example, when the printing drum is stopped where the clamper is short of the clamping position (short run), the position of the stencil master relative to the printing drum is shifted forward in the direction of rotation of the printing drum from the correct position, which results in upward (i.e., toward the leading end of the printing sheet) shift of the printing position on the printed sheet. On the other hand, when the printing drum is stopped where the damper is positioned beyond the clamping position (overrun), the position of the stencil master relative to the printing drum is shifted rearward in the direction of rotation of the printing drum from the correct position, which results in downward (i.e., toward the trailing end of the printing sheet) shift of the printing position on the printed sheet.
Conventionally when wrapping the stencil master, the printing drum is rotated to a position where the damper is positioned in the clamping position (will be referred to as "the reference position", hereinbelow) by a drum motor which drives the printing drum in printing operation. In order to correctly stop the printing drum in the reference position without being affected by inertia of the printing drum, the drum motor is driven at a speed lower than that in the printing operation. However since the drum motor is arranged to rotate the printing drum at a high speed in the printing operation, reduction of the speed of the drum motor is limited and accordingly it has been very difficult to accurately stop the printing drum in the reference position.
This problem may be overcome by use of a low-speed motor separate from the drum motor for driving the printing drum at a high speed in the printing operation. However this approach is disadvantageous in that an additional motor is required and a means for switching the motors is required, which adds to the cost of the printer.